Pre- Cenozoic Stratigraphy and Paleogeography of the Grand Canyon, AZ Abstract Vishnu Basement Rocks

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1 Pre- Cenozoic Stratigraphy and Paleogeography of the Grand Canyon, AZ Amanda D Elia Abstract The Grand Canyon is a geologic wonder offering a unique glimpse into the early geologic history of the North American continent. The rock record exposed in the massive canyon walls reveals a complex history spanning more than a billion years of Earth s history. The earliest known rocks of the Southwestern United States are found in the basement of the Grand Canyon and date back to 1.84 billion years old (Ga). The rocks of the Canyon can be grouped into three distinct sets based on their petrology and age (Figure 1). The oldest rocks are the Vishnu Basement rocks exposed at the base of the canyon and in the granite gorges. These rocks provide a unique clue as to the early continental formation of North America in the early Precambrian. The next set is the Grand Canyon Supergroup, which is not well exposed throughout the canyon, but offers a glimpse into the early beginnings of before the Cambrian explosion. The final group is the Paleozoic strata that make up the bulk of the Canyon walls. Exposure of this strata provides a detailed glimpse into North American environmental changes over nearly 300 million years (Ma) of geologic history. Together these rocks serve not only as an awe inspiring beauty but a unique opportunity to glimpse into the past. Vishnu Basement Rocks The oldest rocks exposed within the Grand Canyon represent some of the earliest known rocks in the American southwest. John Wesley Paul referred to them as the dreaded rock because they make up the walls of some of the quickest and most difficult rapids to navigate. Dating back to 1.84 billion years Figure 1. Stratigraphic column showing the three sets of rocks found in the Grand Canyon, their thickness and approximate ages (Mathis and Bowman, 2006). ago, these basement rocks represent a very different more dynamic period in the North American geologic history than is seen today. These rocks record continental formation of Laurentia, the ancient geologic core of North America, through the aglamation of oceanic island arcs and continental microplates to southern North America (Karlstrom et al., 2012) (Figure 2). These rocks can be separated into three different groups based on age and lithology: Elves Chasm Pluton, Granite Gorge Metamorphic Suite, Zoroaster Plutonic Complex (Karlstrom et al., 2003). Each group has a very different depositional and deformational history. Elves Chasm Pluton The Elves Chasm Gneiss is the oldest known rock in the southwestern United States and

2 forms the basement for the turbidite deposits of the Vishnu Schist. It has a U- Pb zircon age of 1840 ± 1 million years old (Hawkins et al., 1996) and is associated with the Mojave province (Karlstrom et al., 2012). It is dominantly a hornblende- biotite tonalite to quartz diorite (Karlstrom et al., 2003). Its distinct geochemical composition differentiates it from other plutonic rocks of the grand canyon and suggests that it has a less direct genetic relationship to slab subduction, which is seen in the geochemical composition of the younger plutons (Karlstrom et al., 2003). Zircons dating 1.75 Ga are found in the Granite Gorge Metamorphic Suite rocks indicating that they were deposited after the Elves Chasm Pluton. Figure 2. Block models showing the tectonic processes that created the Vishnu Basement Rocks between 1.75 and 1.65 Ga (Karlstrom et al., 2012). Granite Gorge Metamorphic Suite The Granite Gorge Metamorphic Suite rocks are metasedimentary and metavolcanic rocks named the Brahma, Rama and Vishnu schists. These schists have U- Pb zircon ages of 1750 ± 2, 1741 ± 1 and million years old respectively (Karlstrom et al., 2012). The Brahma Schist consists mainly of hornblende- biotite schists and amphibolites. The

3 amphibolites have theolitic character indicating that their origin may be island arc basalts (Karlstrom et al., 2003). Primary features such as pillow structures and volcanic breccia can be seen in various places throughout the canyon also indicating mafic lava interacting with and erupting in water (Karlstrom et al., 2003). The Rama Schist consists mainly of quartzofelspathic schist and gneiss suggesting a felsic to intermediate (terrestrial) volcanic origin (Karlstrom et al., 2003). These two schists are complexly interlayered making non- isotopic dating very difficult. The Vishnu Schist consists of quart- mica schist and pelitic schist interpreted to be metamorphosed sandstones and mudstones that were deposited on the submarine shores of the island arcs (Karlstrom et al., 2003). Some primary features of rhythmic and graded bedding suggest submarine turbidite deposition in generally low energy environments (Karlstrom et al., 2003). The Vishnu Schist overlies the Brahma stratigraphically and has detrital zircon ages earlier than the younger arc plutons. Zoroaster Plutonic Complex The plutons that make up the Zoroaster Plutonic Complex (debate about the lumping of all the different plutons exists, but for simplicity, all younger plutons are categorized under this grouping) record a complex evolution of the crust of the southwestern United States. These plutons fall into two categories based on their age, petrology, geochemistry, and intrusion type. The first are the arc plutons. These plutons date from 1.74 to 1.71 Ga, are mostly composed of gabbro to granodiorite and are geochemically and petrologically interpreted as shallowly emplaced arc plutons (Karlstrom et al., 2003). They have a clac- alkaline granitic composition indicating subduction zone relate arc magmatism (Karlstrom et al., 2012). They intrude the metasedimetnary and metavolcanic rocks making them just younger than them. The second group is syncollisional granites in the form of dike swarms where granitic and pegmatitic magma fluids filled crack systems as magma migrated through the crust (Karlstrom et al., 2012). Many of these dikes are folded stretched and sheared indicating they were emplaced during mountain building and crustal thickening. Their composition is compatible with the partial melting of the previously emplaced plutons and metamorphics (Karlstrom et al., 2012). The age range of these dikes is Ga and coincides with the age of peak metamorphism and contractional deformation (Karlstrom et al., 2003). By 1.65 Ga the Vishnu Basement Rocks were complexly deformed, metamorphosed, and beginning to cool at depths of 10km in the middle crust of the thickened orogen (Karlstrom et al., 2003). This orogeny would not have produced the massive mountain ranges seen in the Himalayas, but were more closely resembling the current Indonesian orogeny (Karlstrom et al., 2012). From 1.6 to 1.4 Ga these rocks would have continued to cool slowly being little affected by North American tectonism. Gradual erosion and beveling of this mountain belt brought the Vishnu basement rocks to the surface approximately 1.2 Ga ago at which point they were buried by the deposition of the Grand Canyon Supergroup forming the Great Angular Unconformity (Karlstrom et al., 2003). Grand Canyon Supergroup The Grand Canyon Supergroup is a series of gently tilted sedimentary and igneous rocks that is exposed in limited outcrops throughout the Canyon and its tributaries. Deposition of these sediments occurred first on the undulating smooth Vishnu surface (Hendricks and Stevenson, 2003). To the west would have been open oceans. As the land subsided the oceans advanced eastward marking the beginning of deposition (Hendricks and Stevenson, 2003). This

4 Supergroup is massively thick (approximately 4000m) and represents pulses of sedimentation accompanied by faulting and followed by extended periods of erosion which brakes the Supergroup into four continuous sedimentary sections: the Unkar Group, the Nankoweap Formation, the Chuar Group and the Sixtymile Formation (Timmons et al., 2012) (Figure 3). Unkar Group The Unkar Group is Figure 3. Stratigraphic section of the Grand Canyon Supergroup showing geochronologic constraints and approximate ages (Timmons et al.,2005 ). approximately 2 km thick and consists of the Hotauta Conglomerate, Bass Limestone, Hakatai Shale, Shinumo Quartzite, Dox Formation and Cardenas Basalt (Hendricks and Stevenson, 2003). This succession contains both river and shallow marine deposits with a significant disconformity between the Hakatai Shale and Shinumo Quartzite (Timmons et al., 2012). It is representative of an east to west transgressive sea with minor sea level fluctuations due to basin filling and subsidence (Hendricks and Stevenson, 2003). The Hotauta Conglomerate marks the beginning of sedimentation with detrital lithics from an area outside the Grand Canyon (Timmons et al., 2005). The Bass limestone was deposited in warm shallow seas and contains stromatalite fossils (Nitecki, 1971) indicating the arrival of life to the Canyon. The Hakatai Shale represents a transitioning environment to a mud- flat shallow marine setting following the transgressive sequence. The Shinumo Quartzite sits unconformably on the Hakatai Shale and represents a high- energy, shoreface environment (Timmons et al., 2012). Following deposition of the Shinumo, the Dox formation was deposited and represents a thick set of four sedimentary members that record a transition from a subaqueous delta, to floodplain, to tidal flat environment (Timmons et al., 2012). Volcanism marked the end of sedimentation with nearly 305m of lava accumulation above the Dox formation (the Cardenas Basalt) (Hendricks and Stevenson, 2003). This volcanism also caused sills to form in the lower Unkar and dikes toe form in the upper Unkar (Timmons et al., 2012). Following the end of volcanism, the Unkar Group was tilted slightly and the basalt was eroded before sedimentation began again in the region (Hendricks and Stevenson, 2003). Nankoweap Formation The Nankoweap formation sits unconformably on the Cardenas Basalt. It consists of a lower member dominated by hematite- cemented sandstone and mudstone and an upper member dominated by siltstone and thin- bedded, fine- grained red bed sandstone (Timmons et al., 2012). The lower member is thought to represent deposition in quiet shallow water confined structurally whereas the upper member is representative of a low to moderate energy shallow marine or lake water environment (Ford and Dehler, 2003). Between these members is a localized deposit of Cardenas Basalt clasts suggesting a period o uplift and erosion between the members (Timmons et al., 2012). Both members are capped by a white, fine- grained quartz

5 arenite (Timmons et al., 2012). The age of this formation is not well known but thought to be closer to the age of the Chuar group (Ford and Dehler, 2003). Chuar Group The Chuar Group represents deposition in a tectonically active marine basin with a fluctuating shoreline (Dehler et al., 2012). Repetitions in the stratigraphy indicate that sea level rose and fell in tempo with global climate changes. The shallow waters of the Chuar Sea were teeming with single- celled life that is seen in the fossils of these rocks (Dehler et al., 2012). As the supercontinent of Rodinia was breaking up, Earth s climate was undergoing glacial and interglacial cycles, there were mass perturbations to the global carbon cycle, and single- celled life was diversifying (Dehler et al., 2012). The Chuar Group consists mainly of mudrock with interbedded sandstone and dolomite. It contains two distinct formations each with various members. The first is the Galeros Formation, which consists mainly of alternating limestones and shales (Ford and Dehler, 2003). The upper Kwagnut Formation consists of a basal red sandstone with overlying stromatalitic dolomites and pisolitic cherts (Ford and Dehler, 2003). The Chuar Group consists of various microfossils including filamentous algal sheets, coccoid algae, acritarchs, vase- shaped microfossils, and stromatolites (Ford and Dehler, 2003). An ash bed overlies the final Chuar Group member and has been dated to 742±6 million years marking that as the end of deposition (Karlstrom et al., 2000). Sixtymile Formation The Sixtymile Formation sits conformably on the Chuar Group but represents a drastic change in environment. Consisting mainly of breccias and sandstones, the Sixtymile Formation represents a mostly terrestrial and syntectonic environment (Ford and Dehler, 2003). Landsliding and slumping in the lower members was followed by colluvial and alluvial sedimentation in upper members (Ford and Dehler, 2003). Following deposition, the strata was uplifted and normal faulting caused fault blocking of the Grand Canyon Supergroup. Erosion occurred during some unconstrained time period and was ended with the deposition of the Tonto Group. Overlying the Grand Canyon Supergroup is the Paleozoic Strata indicating another Great Unconformity between the two contacts spanning 130 Ma. Paleozoic Strata The Paleozoic Sedimentary rocks of the Grand Canyon comprise over 1000m of strata that represent wide- ranging depositional environments (Blakely and Middleton, 2012). These rocks were deposited on stable, cratonic Laurentia following the Proterozoic rifting of Western Laurentia (Blakely and Middleton, 2012). Cambrian through Devonian sediments were deposited on the western passive margin of the continent. Slower rates of subsidence in the east result in thickening to the west (Blakely and Middleton, 2012). These strata provide excellent examples of transgressing and regressing seas. These strata have been broken into 9

6 separate formations and groups that will be further explained below. Figure 4. Locations of the main provinces of North America that may have been the sources for the sediments in the Paleozoic strata of the Grand Canyon (Gehrels et al., 2011) Tonto Group The Tonto Group comprises the Tapeats Sandstone, Bright Angel Shale formations, and Muav Limestone. The Tapeats Sandstone is a medium- to coarse- grained feldspar and quart- rich sandstone representing deposition in intertidal and shallow subtidal zones (Blakely and Middleton, 2012). The Bright Angel Shale is interbedded fine- grained sandstone, siltstone and shale representing a subtidal depositional environment. The Muav Limestone consists of thin- to thick bedded, commonly mottled dolomitic and calcareous mudstone and packstone with first major occurrences of invertebrate fossils in the canyon representing a shallow water depositional environment. The Tonto Group is a classic example of a transgressive sea indicating a fining of sediments up- section as sea levels rise (Middleton and Elliott, 2003). These rocks have ages of Ma with indication of detrital provenance in the Yavapai Province (Gehrels et al., 2011) (Figure 4). Temple Butte Formation The Temple Butte Formation unconformably overlies the Tonto Group representing a disconformity that spans the Ordovician and Silurian (Blakely and Middleton, 2012). The Temple Butte consists mostly of a distinct pale, reddish purple dolomite and sandy dolomite with irregular bedding (Beus, 2003a). This formation is approximately 385 Ma with sediment derivation from the Mazatzal and Yavapai Provinces, midcontinent region, and the Amarillo- Wichita uplift (Gehrels et al., 2011). Redwall Limestone The Redwall Limestone lies unconformably on the Temple Butte Formation where present or on Devonian Strata (Beus, 2003b). It forms the iconic red cliffs throughout the canyon. Consisting of four distinct members of hematite- stained carbonate, the Redwall

7 Limestone Formation represents mostly transgressing and regressing seas in open marine waters (Blakely and Middleton, 2012). Extensive dissolution following deposition led to karst topography (Blakely and Middleton, 2012). The Redwall Limestone is approximately 340 Ma as dated using index fossils (Beus, 2003b). Surprise Canyon Formation The Surprise Canyon Formation is a discontinuous outcrop as clastic and carbonate rocks that fill erosional valleys and karst topography (Beus, 2003b). This formation consists of basal conglomerate and sandstone of fluvial origin, a marine limestone, and a slope- forming marine siltstone (Blakely and Middleton, 2012). Karst topography would have created the valleys some of the fluvial deposits occurred in though the upper members definitively formed in marine environments as seen in the robust fossil evidence (Blakely and Middleton, 2012). The provenance of the sediments of this formation records a major shift as the Appalachian orogen begins (Gehrels et al., 2011). Supai Group The Supai Group consists of four formations, the Watahomigi Formation, Manakacha Formation, Wescogame Formation, and Esplanade Sandstone. Together they represent 30 Ma of depositional strata in the Canyon from 315 to 285 Ma (Blakely, 2003). The Watahomigi Formation consists mainly of mixed limestones and red mudstone deposited in a low- energy, shallow marine and coastal plain environment (Blakely and Middleton, 2012). The Manakacha Formation is comprised of sandstone, mudstone and limestone indicating a low- lying coastal plain or shallow marine depositional setting (Blakely and Middleton, 2012). The Wescogame Formation is similar to the Manakacha, but a decrease in marine fossils and increase in sand and mud indicate encroachment of continental environments (Blakely and Middleton, 2012). The final member, the Esplanade Sandstone consists mainly of cross- stratified sandstone most likely deposited in eoilan dune and sand sheet environments (Blakely and Middleton, 2012). Detrital zircon studies indicate that the provenance for the sediment of this group is coming from both the Ancestral Rocky Mountain and Appalachian orogenies (Gehrels et al., 2011). Hermit Formation The Hermit Formation consists mainly of slope- forming, reddish brown siltstone, mudstone, and very fine- grained sandstone (Blakely, 2003). The unit formed on a broad coastal plain mostly in a fluvial environment though there is some loess and scattered dune deposits (Blakely and Middleton, 2012). The Hermit formation is approximately 280 Ma, and sediment for this formation came mainly from the Appalachian orogeny and a lesser proportion from the Ancestral Rocky Mountains (Gehrels et al., 2011). Coconino Sandstone The Coconino Sandstone consists almost exclusively of bright, pale yellow large- scale cross- stratified sandstone (Blakely and Middleton, 2012). This formation records the southward advancement of large Sahara- like dunes making up a massive dessert at the time, 275Ma (Middleton et al., 2003). It lies disconformably over the Hermit Formation (Middleton et al., 2003). Due to a lack of actual fossils, this formation contains many trace fossils from both vertebrates and invertebrates (Middleton et al., 2003). As is the case with all of the Permian strata, sediment for this formation came mainly from the Appalachian orogeny and a lesser proportion from the Ancestral Rocky Mountains (Gehrels et al., 2011). Toroweap Formation

8 The Toroweap Formation lies conformably on the Coconino Sandstone and consists of limestone, dolostone, sandstone, bedded gypsum, and sandy mudstone (Turner, 2003). These shifts in rock type coincide with shifts from a shallow marine to sabkha, restricted coastal plain, and local continental environments during times of transgressing and regressing seas (Blakely and Middleton, 2012; Turner, 2003). The Toroweap is dated at 273 Ma, and sediment for this formation came mainly from the Appalachian orogeny and a lesser proportion from the Ancestral Rocky Mountains (Gehrels et al., 2011). Kaibab Limestone The final Paleozoic unit, the Kaibab Limestone (Hopkins and Thompson, 2003) consists mostly of cherty limestone and dolostone deposited in both open- marine and restricted- marine environments. It is approximately 270Ma and sediments originate mostly from the Appalachian orogeny (Gehrels et al., 2011). Concluding Remarks The stratigraphy of the Grand Canyon has provide geologists a rare opportunity to study the geologic history of southwestern North America on a grand scale. However, there is still much to be discovered. Many of the formations within this canyon lie unconformably meaning there are large gaps in the known history. There has been research done to compare the strata found in the Canyon with other exposures found elsewhere in the southwest that has provided some clues to this missing history. However, some of it, such as the missing 400 Ma between the Vishnu rocks and the Grand Canyon Supergroup, may forever remain a mystery. Research continues to be done in this region to better understand not only rocks found within the canyon, but also the complex geologic story the canyon tells. Literature Cited Beus, S.S., 2003a. Temple Butte Formation in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2 nd Edition, p Beus, S.S., 2003b. Redwall limestone and Surprise Canyon Formation in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2 nd Edition, p Blakely, R.C., Supai Group and Hermit Formation in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2 nd Edition, p Blakely, R.C., and Middleton, L.T., Geologic history and paleogeography of Paleozoic and early Mesozoic sedimentary rocks, eastern Grand Canyon, Arizona, in Timmons, J.M., and Karlstrom, K.E., eds., Grand Canyon Geology: Two Billion Years of Earth s History: Geological Society of America Special paper 489, p Dehler, C.M., Porter, S.M., and Timmons, J.M., The Neoproterozoic Earth system revealed from the Chuar Group of the Grand Canyon, in Timmons, J.M., and Karlstrom, K.E., eds., Grand Canyon Geology: Two Billion Years of Earth s History: Geological Society of America Special paper 489, p

9 Ford, T.D., and Dehler, C.M., Grand Canyon Supergroup: Nankoweap Formation, Chuar Group, and Sixtymile Formation, in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2 nd Edition, p Gehrels, G.E., Blakey, R., Karlstrom, K.E., Timmons, J.M., Dickinson, B., and Pecha, M., 2011, Detrital zircon U/Pb geochronology of Paleozoic strata in the Grand Canyon, Arizona., Lithosphere 3 (3), p Hawkins, D.P., Bowring, S.A., Ilg, B.R., Karlstrom, K.E., and Williams, M.L., U- Pb geochronologic constraints on the Paleoproterozoic crustal evolution of the Upper Granite Gorge, Grand Canyon, Arizona: Geological Society of America Bulletin, v. 108, p Hendricks, J.D., and Stevenson, G.M., Grand Canyon Supergroup: Unkar Group, in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2nd Edition, p Hopkins, R.L., and Thompson K.L., 2003 Kaibab Formation, in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2nd Edition, p Kalrstrom, K.E., Ilg, B.R., Hawkins, D., Williams, M.L., Dummond, G., Mahan, K., and Bowring, S.A., Vishnu basement rocks of the Upper Granite Gorge: Continent formation 1.84 to 1.66 billion years ago, in Timmons, J.M., and Karlstrom, K.E., eds., Grand Canyon Geology: Two Billion Years of Earth s History: Geological Society of America Special paper 489, p Karlstrom, K.E., Ilg, B.R., Williams, M.L., Hawkins, D.P., Bowring, S.A., and Seaman, S.J., Paleoproterozoic Rocks of the Granite Gorges, in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2nd Edition, p Karlstrom, K.E., Bowring, S.A., Dehler, C.M., Knoll, A.H., Porter, S.M., Des Marais, D.J., Weil, A.B., Sharp, Z.D., Geissman, J.W., Elrick, M.B., Timmons, J.M., Crossey, L.J., and Davidek, K.L., Chuar Group of the Grand Canyon: Record of breakup of Rodinia, associated change in the global carbon cycle, and ecosystem expansion by 740 Ma. Geology 28, p Mathis, A., and C. Bowman, The Grand Age of Rocks: The Numeric Ages for Rocks Exposed within Grand Canyon. Available at Middleton L. T., and D. K. Elliott, Tonto Group, in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2nd Edition, p Middleton L. T., D. K. Elliott, and Morales, M., Coconino Sandstone in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2 nd Edition, p

10 Nitecki, M.H., Pseudo- organic structures from the Precambrian Bass Limestone in Arizona: Geology, v. 23, p Timmons, J.M., Karlstrom, K.E., Heizler, M.T., Bowring, S.A., Gehrels, G.E., and Crossey, L.J., Tectonic inferences from the ca Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis. Geological Society of America Bulletin 117 (11-12), p Timmons, J.M., Bloch, J., Fletcher, K., Karlstrom, K.E., Heizller, M., and Crossey, L.J., The Grand Canyon Unkar Group: Mesoproterozoic basin formation in the continental interior during supercontinent assembly, in Timmons, J.M., and Karlstrom, K.E., eds., Grand Canyon Geology: Two Billion Years of Earth s History: Geological Society of America Special paper 489, p Turner, C.E., Toroweap Formation, in Grand Canyon Geology, eds. S.S. Beus and M. Morales. New York: Oxford University Press, 2 nd Edition, p